INTRODUCTION

Torsional vibration is an oscillatory angular motion causing twisting in the shaft of a system; the oscillatory motion is superimposed on the steady rotational motion of a rotating/reciprocating machine. Even though the vibration cannot be detected without special measuring equipment, its amplitude can be destructive. For example, gear sets that alter speeds of power transmission systems transmit the vibration to the casing. Similarly, slider crank mechanisms in engines and compressors convert torques to radial forces that are discernable to human perception but are not measurable because of the insensitivity of test equipment and background noise. If gearboxes or reciprocating machines are part of a drive train, excess noise and vibration can indicate trouble. Standards and measurement methods dealing with acceptable magnitudes of radial vibration are provided in Chap. 19.

Motion is rarely a concern with torsional vibration unless it affects the function of a system. It is stresses that affect the structural integrity and life of components and thus determine the allowable magnitude of the torsional vibration. Torsional vibratory motions can produce stress reversals that cause metal fatigue. Components tolerate less reversed stress than steady stress. In addition, stress concentration factors associated with machine members decrease the effectiveness of load-bearing materials.

Figure 38.1 illustrates the twisting of a shaft of an electric motor-compressor system. The torsional mode shape associated with the first torsional natural frequency is shown in Fig. 38.2. A coupling in the power train allows for misalignment in the assembly. The mode shape shows...